The present invention relates to a process for coating a substrate. More particularly, the invention relates to coating a substrate with a iron oxide-containing material, preferably a magnetic iron oxide-containing material.
An application where substrates with coatings, e.g., magnetic conductive coatings, find particular usefulness is memory cores, linear, power and recording head application, magnets and heating.
In many of the above-noted applications it would be advantageous to have a magnetic iron oxide which is substantially uniform, has high permeability, and has good chemical properties, e.g., morphology, stability, etc.
A number of techniques may be employed to provide conductive iron oxide coatings on substrates. Most ferrites are prepared as ceramic materials by standard ceramic processing. In this process the constituent raw materials, oxides, hydroxides, or carbonates, are weighed and first milled in a steel mill using steel balls as the milling media and water as the carrier. During milling, the raw materials are mixed to yield a homogeneous mixture. Other mixing methods may also be employed such as dry mixing of raw materials. The milling gives uniform mixing and results in some size reduction leading to better reactivity in the calcining step. In the calcining (sometimes called presintering) reaction, the raw materials are heated to 800.degree. to 1300.degree. C. and form the ferrite compound. The carbonates decompose and react by solid-state diffusion to form the final compound.
In the case of the nickel-zinc-spinel ferrites, the powder is calcined at a temperature of ca 1027.degree. C. to yield an agglomerated, friable powder that is essentially 100% converted to the spinel phase. However, in the case of the manganese-zinc-ferrites, the calcining conditions are such that the material is 50-85% converted to spinel. Time and temperature are the most important control parameters in the calcining step.
The purpose of this millings is to further homogenize the material and to reduce the particle size to permit subsequent pressing and sintering. The milling itself can be carried out in a variety of ways, for example, wet-ball mill with steel balls in a manner analogous to the first milling. The main objective is to get a finely divided powder that can be slurried and spray dried.
Following the second milling, the material must be granulated so that it will be free flowing and can be dry pressed into the desired shape. A method for producing ferrite powder is to add a binder such as poly(ethylene glycol) or poly(vinyl alcohol) at 1-4 wt % and sufficient water for form a slurry that is about 65-70 wt % ferrite. The slurry is spray dried to yield a dry powder consisting of small spherical particles having a narrow size distribution.
Very thin parts, such as used in memory cores, amy be formed by tape casting followed by punching the desired shape. Parts that have a high length-to-diameter ratio may be formed by either extrusion or by isostatic pressing.
In the sintering process, the ceramic material is densified and the final magnetic properties are developed. Some materials such as the iron-deficient nickel-zinc=ferrites and the M-type hexagonal ferrites may be fired in air because all the cations exist at their highest valence state. However, with the manganese-zinc ferrites the amount of ferrous iron (Fe.sup.2+) in the crystal lattice is controlled. Typical temperatures for the sintering zone are in the range of 1275-1450.degree. C.; sintering time may range from 20 minutes to 12 hours.
The next zone in the kiln is called the anneal or equilibration zone, where the temperature is dropped to 1000.degree.-1300.degree. C. and the oxygen content of the atmosphere is lowered by the introduction of nitrogen gas. At this elevated temperature the ferrite equilibrates quickly with the atmosphere, and the desired ferrous iron level is established. Following the annealing step, the parts are cooled as rapidly as possible and the oxygen content of the atmosphere is reduced still further.
In an attempt to improve chemical homogeneity, a wet-chemical process was designed in which an aqueous solution was prepared containing the metal cations. Addition of a strong base (eg, NaOH) precipitated an intermediate hydroxide which was subsequently oxidized by bubbling air through the suspension. The results was a homogeneous fine-particle ferrite. A similar type of process used an ammonium bicarbonate-ammonium hydroxide mixture as the precipitating agent followed by conventional calcining.
The preparation of ferrite compounds by the cryochemical method has also been investigated. In this technique, an aqueous solutio is sprayed into a chilled liquid (eg, hexane) where the droplets freeze into beads ca) 0.4 mm diameter. These pellets are removed from the liquid and placed in a freeze dryer where the moisture is removed by sublimation. The resultant pellets are converted to the spinel by calcining.
The preparation of the hexagonal ferrites by wet-chemical precipitation, topotactic reaction, and fluidized-bed reaction has been investigated. However, the most common method is standard ceramic processing.
Critical areas of process control in the conventional type processing are the composition and the presintering conditions. The calcining step is especially critical because it determines to a large extent the properties of the magnet after sintering. At a typical calcining temperature of 1300.degree. C. the material reacts completely to form the hexagonal phase. If calcining takes place at a lower temperature, the magnetic properties are not affected adversely but the calcined material is too soft and the subsequent milling step which gives a very fine particle size. This leads to difficulty in pressing and a very high shrinkage during sintering. If, on the other hand, the sintering temperature is too high, the particles are too hard and the particle size after milling is rather coarse. Although this does not cause a pressing problem, after sintering the particles are too large and the shrinkage and coercive force are both too low.
After calcining the material must be milled to reduce the particle size to the range of 1 um in order to obtain single-domain properties.
Fabrication of the milled powder into parts can take place by a number of methods depending on the degree of magnetic alignment desired. For the lowest-grade material, the milled powder is spray dried and then dry pressed into the required shape. In these materials, the individual particles are randomly aligned with respect to each other, resulting in a isotropic magnet in which the magnetic properties are the same in all directions.
Anisotropic magnets are prepared by dry or wet pressing the material in the presence of an external magnetic field which causes the individual magnetic particles to align themselves with that field. The dry-pressing technique is quite similar to that used for preparing isotropic magnets, except that pressing takes place in the presence of a magnetic field.
Wet pressing, gives the highest degree of alignment with the field because the individual particles are much freer to rotate under its influence. When alignment is essentially complete, the water is removed by applying a vacuum to the die cavity, and a very fine filter paper prevents the powder from being pulled out with the water.
Sintering of dry-pressed parts can take place immediately after forming. However, wet-pressed parts must be carefully dried to remove most of the residual moisture before being placed in the kiln. Drying under controlled conditions may take from 10 to 200 hours, depending on size and shape.
The pressed parts are sintered in the air at 1125.degree.-1375.degree. C. to yield a dense ceramic material. In order to minimize the grain growth that occurs during sintering, the firing temperature is kept as low as possible.
Conventional processing has been used for the preparation of powder for follow on consolidation into final shapes. Such processing has not been directed at or concerned with thin and/or thick films and a wide variety of inorganic substrates, the Novel components and articles produced or the unique properties of such coated components in a wide variety of applications.
One of the preferred substrates for use in certain magnetic mechanical devices are inorganic substrate, in particular flakes, spheres, fibers and other type particles.